Seal assemblies are known to be used for providing a rotating type of seal between a stator fixed with respect to an axis and a shaft rotating with respect to the same axis.
Seal assemblies are able to prevent leakage of a first lubricating fluid, typically oil, contained in a first chamber to a second chamber, which is occupied by a second fluid, typically air.
More specifically, seal assemblies are known that are formed by:                a first fixed ring mounted on the stator; and        a second rotating ring mounted on the rotor.        
The first ring and the second ring are axially pressed together, for example by elastic means, and respectively have a first surface and a second surface facing each other and placed in contact against each other.
More precisely, sealing is ensured by the pressure existing between the first surface and the second surface.
Seal assemblies capable of providing a rotating hydrodynamic seal are also known, as disclosed in U.S. Pat. No. 6,257,589 for example.
These seal assemblies comprise:                a first chamber, which contains oil;        a second chamber, which contains air; and        a plurality of grooves made on the second surface of the second ring and known in the industry as hydropads.        
In greater detail, the first chamber is arranged radially outermost with respect to the second chamber and with reference to the shaft's axis of rotation.
The first surface of the first ring is partly facing the first chamber, while the grooves are partly open to the inside of the second chamber.
The second ring is radially interposed between the first chamber and the second chamber.
The grooves extend radially from the axis of rotation of the shaft, are angularly equidistant about the ring's axis of rotation and are blind in a direction parallel to the axis of rotation.
The grooves alter the thickness of the second ring in a circumferential direction with respect to the axis of rotation. Due to the rotation of the second ring, this variation in thickness creates overpressure inside the air film contained therein and trapped between the first surface and the second surface.
In other words, the second rotating ring fitted with grooves behaves like a sliding shoe of different thickness in translation with respect to a viscous fluid.
This overpressure creates a kind of “air barrier”, which impedes oil from leaking from the first chamber.
More precisely, each groove is delimited by:                a first edge, which is arranged radially internal with respect to the axis and is shaped like a circumferential arc; and        a second edge, which is arranged radially external with respect to the axis, is shaped like a circumferential arc and faces the first edge.        
Each groove is also delimited by a third edge and a fourth edge, which both extend between the first edge and the second edge.
More precisely, the third edge and the fourth edge face each other and circumferentially delimit the groove.
Furthermore, the third and fourth edges are shaped like segments of a straight line.
Although they are efficient, the aforementioned seal assemblies incorporating grooves can be improved.
In greater detail, a need is felt within the industry to optimize the reliability and operating life of the seal assembly.
More precisely, a need is felt within the industry to increase the overpressure effect in the vicinity of the first edge, i.e. close to the second chamber.
A need is also felt for using the grooves not just for creating an overpressure air-barrier effect, but also for returning any drops of oil entering the groove back inside the first chamber.
Finally, a need is felt for controlling the amount of lubricating fluid that can wet the contact surface between the first ring and the second ring, so as to ensure adequate lubrication and, in consequence, adequate reliability of the seal assembly.
WO95/29353 and EP-A-0582445 disclose a sealing ring with only one series of circumferentially spaced grooves. In particular, the radially outer edges and the radially inner edges of all the grooves lie on only one radially outer circumference and on only one radially inner circumference respectively.
U.S. Pat. No. 6,152,452 disclose a sealing ring with two series of first and second spaced grooves, which are alternate with one another in a circumferential direction and staggered from each other in a radial direction.
In particular, the first grooves and the second grooves are bounded by spiral edges and are shaped differently from one another. Still more precisely, the radial size of the first grooves arranged radially external is greater than the radial size of the second grooves.